1. Field of the Invention
This invention relates to a liquid electrophotographic developing device for developing a photoconductive material charged and exposed by the use of a liquid toner.
2. Background Information
A liquid electrophotographic developing device is proposed to develop a photoconductive material (such as an electro-lithograph plate which is produced by forming a photoconductive layer made of ZnO and binder on a base material water-proofed and made conductive) which bears an electrostatic latent image after being charged and exposed to an image. Development is performed by applying a liquid toner to the photoconductive material so as to make the electrostatic latent image borne thereby visible.
Specifically, when two electrodes made of metallic plates and disposed in closely-spaced relation are directly connected together or when bias voltage is applied to the two electrodes in order to minimize fogging in the image, toner flowing between the two electrodes develops the photoconductive material which is also traveling between the two (developing and back) electrodes. In conventional developing devices, however, toner particles tend to electro-deposit on the two electrodes, especially on the back electrode, so that the photoconductive material is soiled upon contact with the electrodes during development. The contact of the photoconductive material with the electrodes has been avoided by increasing the distance between the developing electrode and the back electrode. However, if the distance between the electrodes is increased, the electric field between the developing electrode and the photoconductive material weakens, thereby lowering the efficiency of development which causes insufficient development resulting in visual images of poor reproducibility. An ordinary way to prevent the photoconductive material from being soiled is to stretch a wire material such as gut around the back electrode.
Japanese Patent Application Laid-Open No. 59-185373 teaches a liquid developing device in which a groove is formed in a back electrode. An insulating guide is then disposed between the back electrode and a photoconductive material, so that the photoconductive material is prevented from coming into contact with the back electrode so as to avoid the soiling of the photoconductive material.
In this prior art, however, when the back electrode is soiled and thus the efficiency of development is lowered, the developing device must be stopped to wash the back electrode. The work of washing toner from the back electrode is difficult in the case where a wire material such as gut is provided around the back electrode.
Japanese Patent Publication No. 50-38943 teaches a liquid developing device in which a liquid toner is caused to flow vigorously, so that a strong stream of liquid toner prevents the photoconductive material from coming in contact with the back electrode and washes off the toner adhering to the back electrode.
Japanese Patent Publication No. 59-8832 teaches a liquid developing device in which a liquid toner is vigorously introduced between the photoconductive material and the developing electrode and between the photoconductive material and the back electrode at flow speeds higher than the speed of the photoconductive material passing between the electrodes to prevent the photoconductive material from coming in contact with the metallic plates and to wash off the toner adhering to the back electrode.
In the liquid developing device taught in Japanese Patent Publication No. 50-38943, when development is carried out for a long time, toner particles adhere gradually to the back electrode disposed on the back side of the photoconductive material thereby soiling the back electrode. If the toner adheres to the back electrode, a non-conductive film is formed on the surface of the back electrode, so that the function of the electrode is deteriorated causing the efficiency of development to be lowered. Therefore, the proper amount of toner does not adhere to the photoconductive material and good development cannot be achieved; thus, the developing device must be stopped periodically to wash the back electrode.
In the liquid developing device taught in Japanese Patent Publication No. 59-8832, although the rate at which toner particles adhere to the back electrode can be decreased, the liquid toner adheres to the back electrode in long-term processing; thus, the back electrode must be washed as is the above case.
To solve the foregoing problems, Japanese Patent Application Laid-Open No. 1-260463 teaches a direct-powered developing system in which a conductor such as a brush is disposed so as to come into contact with the back surface of a photoconductive material, and to perform development, a liquid toner is supplied between the photoconductive material and the developing electrode under the condition that the conductor or back electrode and a base material of the photoconductive material are in direct contact. In this system, since toner particles never electro-deposit on the back electrode, the back electrode is perfectly prevented form being soiled through electro-deposition.
In this developing system, however, if the conductor is kept in a wet state or holds a developing solution adhering thereto for a long time (e.g. from the end of developing work to the beginning of developing the next day), the liquid toner adhering to the conductor dries and solidifies, so that a non-conductive coating is formed to lower the efficiency of development. To avoid such a defect, the developing electrode and the like must be washed after the end of the developing work or before the beginning of next development. This work is troublesome.
In this regard, Japanese Patent Application Laid-Open No. 64-38771 teaches a system in which to prevent soiling, a liquid toner is caused to continuously flow toward electrodes to prevent the drying and solidifying of the liquid toner in case a liquid developing device is to be stopped for a comparatively short time; or, a washing solution is caused to automatically flow in case the developing device is stopped for a long time. That is, to prefectly prevent the electrodes from being soiled by electro-deposition, drying and solidifying of the toner, a toner circulating means for short-term stoppage and an automatic washing means for long-term stoppage are incorporated in the direct-powered developing device.
However, in the liquid developing device taught in Japanese Patent Application Laid-Open No. 64-38771, the liquid toner flows toward the developing electrode even while development is not carried out. This results in a large degree of vaporization so that the concentration of the liquid toner varies. Specifically, the toner concentration varies largely between where a large volume of work is contiguously carried out and where a small volume of work is intermittently done; thus, the amount of toner adhering to the photoconductive material bearing an electrostatic latent image varies in long-term processing. Such a difference in toner concentration will be corrected by changing the concentration of a supplementary toner between contiguous large-volume processing and intermittent small-volume processing; but, this makes control complicated or non-practicable. Another system was taught in which with a toner of high concentration and a carrier solution prepared, they are automatically weighed and introduced in response to each measurement of the toner concentration to bring about a proper concentration; but, this system was rarely practiced because the device is complicated in structure and development is influenced due to the reliability of meters and the like.
As described above, in any of the conventional liquid developing devices and systems, toner particles adhere to the back electrode or the like, so that a non-conductive coating is formed to lower the efficiency of development; thus, the device must be washed frequently to recover an intended efficiency of development, making maintenance troublesome. On the other hand, in the method free of the above defects, the concentration of the liquid toner varies due to vaporization. That is, toner concentration varies between contiguous large-volume processing and intermittent small-volume processing to change a final quality. Consequently, a very complicated control must be incorporated to overcome the above.